Git Introduction

Jens Lechtenbörger

VM OER 2019/2020

Contents

1 Introduction 1

2 Git Concepts 3

3 Git Basics 5

4 GitLab 12

5 Aside: Lightweight Markup Languages 13

6 Conclusions 13

1 Introduction

1.1 Learning Objectives

� Explain bene�ts of version control systems (e.g., in the context of univer-sity study) and contrast decentralized ones with centralized ones

� Explain states of �les under Git and apply commands to manage them

� Explain Feature Branch Work�ow and apply it in sample scenarios

� Edit simple Markdown documents

1.2 Core Questions

� How to collaborate on shared documents as distributed team?

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Figure 1: �Magit screenshot� under CC0 1.0; from GitLab

� Consider multiple people working on multiple �les

* Potentially in parallel on the same �le

* Think of group exercise sheet, project documentation, sourcecode

� How to keep track of who changed what why?

� How to support uni�ed/integrated end result?

1.3 Version Control Systems (VCSs)

� Synonyms: Version/source code/revision control system, source code man-agement (SCM)

� Collaboration on repository of documents

� Each document going through various versions/revisions

* Each document improved by various authors

· April 2012, Linux kernel 3.2: 1,316 developers from 226 com-panies

1.3.1 Major VCS features

� VCS keeps track of history

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� Who changed what/why when?

Figure 2: �Image� under CC0 1.0; rotated from Pixabay

� VCS supports merging of versions into uni�ed/integrated version

� Integrate intermediate versions of single �le with changes by multipleauthors

� Copying of �les is obsolete with VCSs

� Do not create copies of �les with names such as Git-Intro-Final-1.1.txtor Git-Intro-Final-reviewed-Alice.txt

* Instead, use VCS mechanism, e.g., use tags with Git

2 Git Concepts

2.1 Git: A Decentralized VCS

� Various VCSs exist

� E.g.: Git, BitKeeper, SVN, CVS

* (Color code: decentralized, centralized)

� Git created by Linus Torvalds for the development of the kernel Linux

� Reference: Pro Git book

Figure 3: �Git Logo� by Jason Long under CC BY 3.0; from git-scm.com

� Git as example of decentralized VCS

* Every author has own copy of all documents and their history

* Supports o�ine work without server connectivity

· Of course, collaboration requires network connectivity

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2.2 Key Terms: Fork, Commit, Push, Pull

� Fork/clone repository

Figure 4: �Image� under CC0 1.0; derived from Pixabay

� Create your own copy of a repository

� Commit (aka check-in)

Figure 5: �Image� under CC0 1.0; derived from Pixabay

� Make (some or all) changes permanent; announce them to versioncontrol system

� Push: Publish (some or all) commits to remote location

� Fetch (pull): Retrieve commits from remote location (also mergethem)

2.3 Key Terms: Branch, Merge

� Branches

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Figure 6: �Git Branches� by Atlassian under CC BY 2.5 Australia; dimensionattributes added, from Atlassian

� Alternative versions of documents, on which to commit

* Without being disturbed by changes of others

* Without disturbing others

· You can share your branches if you like, though

� Merge

� Combine changes of one branch into another branch

* May or may not need to resolve con�icts

2.4 Git explained by Linus Torvalds

� Video at archive.org (Tech Talk, 2007, by Google Talks under CC BY-NC-SA 3.0)

� Total length of 84 minutes, suggested viewing: 7:40 to 29:00

2.4.1 Review Questions

Prepare answers to the following questions

� What is the role of a VCS (or SCM, in Torvald's terminology)?

� What di�erences exist between decentralized and centralized VCSs?

� By the way, Torvald distinguishes centralized from distributed SCMs.I prefer �decentralized� over �distributed�. You?

3 Git Basics

3.1 Getting Started

� Install Git

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� You may use Git without a server

� Run git init in any directory

* Keep track of your own �les

� By default, you work on the master branch

* master is not more special than any other branch you may create

3.2 Git with Remote Repositories

� Download �les from public repository: clone

� git clone https://gitlab.com/oer/cs/programming.git

* Later on, git pull merges changes to bring your copy up todate

� Contribute to remote repository

� Create account �rst

* Typically, ssh key pairs (next slide) are used for strong authen-tication; register under your account's settings

� Fork project

* either in GUI

* or clone your copy, add upstream

3.2.1 Secure Shell

� Secure Shell (ssh): network protocol for remote login with end-to-endencryption based on asymmetric cryptography

� Popular free implementation: OpenSSH

* Tool to create key pair: ssh-keygen

� Instructions on GitLab

� (In case you are a�ected, note that Git Bash on Windows is men-tioned)

3.3 First Steps with Git

� Prerequisites

� You installed Git

� You performed the First time Git setup

� Part 0

� Create repository or clone one

* git clone https://gitlab.com/oer/cs/programming.git

* Creates directory programming

· Change into that directory

· Note presence of sub-directory .git (with Git meta-data)

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3.3.1 Part 1: Inspecting Status

� Execute git status

� Output includes current branch (master) and potential changes

� Open some �le in text editor and improve it

� E.g., add something to Git-Introduction.org

� Create a new �le, say, test.txt

� Execute git status again

� Output indicates

* Git-Introduction.org as not staged and modi�ed

* test.txt as untracked

* Also, follow-up commands are suggested

· git add to stage for commit

· git checkout to discard changes

3.3.2 Part 2: Staging Changes

� Changes need to be staged before commit

� git add is used for that purpose

� Execute git add Git-Introduction.org

� Execute git status

* Output indicates Git-Introduction.org as to be committedand modi�ed

� Modify Git-Introduction.org more

� Execute git status

� Output indicates Git-Introduction.org as

* To be committed and modi�ed

· Those are your changes added in Part 1

* As well as not staged and modi�ed

· Those are your changes of Part 2

3.3.3 Part 3: Viewing Di�erences

� Execute git diff

� Output shows changes that are not yet staged

* Your changes of Part 2

� Execute git diff --cached

� Output shows changes from last committed version

* All your changes

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� Execute git add Git-Introduction.org

� Execute both diff variants again

� Lots of other variants exits

* Execute git help diff

* Similarly, help for other git commands is available

3.3.4 Part 4: Committing Changes

� Commit (to be committed) changes

� Execute git commit -m "<what was improved>"

* Where <what was improved> should be meaningful text

* 50 characters or less

� Execute git status

� Output no longer mentions Git-Introduction.org

* Up to date from Git's perspective

� Output indicates that your branch advanced; git push suggested forfollow-up

� Execute git log

� Output indicates commit history

� Note your commit at top

3.3.5 Part 5: Undoing Changes

� Undo premature commit that only exists locally

� Execute git reset HEAD~

* (Don't do this for commits that exist in remote places)

� Execute git status and git log

* Note that state before commit is restored

* May applied more changes, commit later

� Undo git add with git reset

� Execute git add Git-Introduction.org

� Execute git reset Git-Introduction.org

� Restore committed version

� Execute git checkout -- <file>

� Warning: Local changes are lost

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3.3.6 Part 6: Stashing Changes

� Save intermediate changes without commit

� Execute git stash

� Execute git status and �nd yourself on previous commit

� Apply saved changes

� Possibly on di�erent branch or after git pull

� Execute git stash apply

* May lead to con�icts, to be resolved manually

3.3.7 Part 7: Branching

� Work on di�erent branch

� E.g., introduce new feature, �x bug

� Execute git checkout -b testbranch

* Option -b: Create new branch and switch to it

� Execute git status and �nd yourself on new branch

* With uncommitted modi�cations from master

* Change more, commit on branch

* Later on, merge or rebase with master

� Execute git checkout master and git checkout testbranch toswitch branches

3.3.8 Review Questions

� As part of First Steps with Git, git status inspects repository, in par-ticular �le states

� Recall that �les may be untracked, if they are located inside a Gitrepository but not managed by Git

� Other �les may be called tracked

� Prepare answers to the following questions

� Among the tracked �les, which states can you identify from thedemo? Which commands are presented to perform what state tran-sitions?

� Optional: Draw a diagram to visualize your �ndings

3.4 Merge vs Rebase

� Merge and rebase unify two branches

� Illustrated subsequently

� Same uni�ed result

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3.4.1 Merge vs Rebase (1)

� Suppose you created branch for new feature and committed on thatbranch; in the meantime, somebody else committed to master

Figure 7: �A forked commit history� by Atlassian under CC BY 2.5 Australia;from Atlassian

3.4.2 Merge vs Rebase (2)

� Merge creates new commit to combine both branches

� Including all commits

� Keeping parallel history

Figure 8: �Merging� by Atlassian under CC BY 2.5 Australia; from Atlassian

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3.4.3 Merge vs Rebase (3)

� Rebase rewrites feature branch on master

� Applies commits of feature on master

� Cleaner end result, but branch's history lost/changed

Figure 9: �Rebasing� by Atlassian under CC BY 2.5 Australia; from Atlassian

3.5 Git Work�ows

� Team needs to agree on git work�ow

� Several alternatives exist

� Feature Branch Work�ow may be your starting point

� Clone remote repository

� Create separate branch for each independent contribution

* E.g., bug �x, new feature, improved documentation

* Enables independent work

� Once done, push that branch, create pull/merge request, receive feed-back

* Pull/Merge request: special action asking maintainer to in-clude your changes

* Maintainer may merge branch into master

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3.5.1 Sample Commands

git clone <project-URI>

# Then, later on retrieve latest changes:

git fetch origin

# See what to do, maybe pull when suggested in status output:

git status

git pull

# Create new branch for your work and switch to it:

git checkout -b nameForBranch

# Modify/add files, commit (potentially often):

git add newFile

git commit -m "Describe change"

# Push branch:

git push -u origin nameForBranch

# Ultimately, merge or rebase branch nameForBranch into branch master

git checkout master

git merge nameForBranch

# If conflict, resolve as instructed by git, commit. Finally push:

git push

4 GitLab

4.1 GitLab Overview

� Web platform for Git repositories

� https://about.gitlab.com/

� Free software, which you could run on your own server

� Manage Git repositories

� Web GUI for forks, commits, pull requests, issues, and much more

� Noti�cations for lots of events

* Not enabled by default

� So-called Continuous Integration (CI) runners to be executed uponcommit

* Based on Docker images

* Build whatever needs building in your project (executables, doc-umentation, presentations, etc.)

4.2 GitLab in Action

� In class

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5 Aside: Lightweight Markup Languages

5.1 Lightweight Markup

� Markup: �Tags� for annotation in text, e.g., indicate sections and head-ings, emphasis, quotations, . . .

� Lightweight markup

� ASCII-only punctuation marks for �tags�

� Human readable, simple syntax, standard text editor su�cient toread/write

� Tool support

* Comparison and merge, e.g., three-way merge

* Conversion to target language (e.g. (X)HTML, PDF, EPUB,ODF)

· Wikis, blogs

· pandoc can convert between lots of languages

5.2 Markdown

� Markdown: A lightweight markup language

� Every Git repository should include a README �le

� What is the project about?

� Typically, README.md in Markdown syntax

� Learning Markdown

� In-browser tutorial (source code under MIT License)

� Cheatsheet (under CC BY 3.0)

5.3 Org Mode

� Org mode: Another lightweight markup language

� My favorite one

� For details see source �le for this presentation as example

6 Conclusions

6.1 Summary

� VCSs enable collaboration on �les

� Source code, documentation, theses, presentations

� Decentralized VCSs such as Git enable distributed, in particular o�ine,work

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� Keeping track of �les' states

* With support for subsequent merge of divergent versions

� Work�ows may prescribe use of branches for pull requests

� Documents with lightweight markup are particularly well-suited for Gitmanagement

License Information

This document is part of a larger course. Source code and source �les areavailable on GitLab under free licenses.

Except where otherwise noted, this work, �Git Introduction�, is © 2018,2019 by Jens Lechtenbörger, published under the Creative Commons licenseCC BY-SA 4.0.

No warranties are given. The license may not give you all of thepermissions necessary for your intended use.

In particular, trademark rights are not licensed under this license. Thus,rights concerning third party logos (e.g., on the title slide) and other (trade-)marks (e.g., �Creative Commons� itself) remain with their respective holders.

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